Anthrax is an acute infectious disease caused by the spore-forming bacterium Bacillus anthracis. Anthrax most commonly occurs in wild and domestic lower vertebrates (e.g., cattle, sheep, goats, camels, antelopes, or other herbivores), but it can also occur in humans when they are exposed to infected animals or tissues from infected animals. In addition, Bacillus anthracis is one of the most important pathogens on the list of bioterrorism threats. The human LD50 for inhalational exposure is about 8,000 to 40,000 spores, or one deep breath at site of release.
Anthrax infection can occur in at least three forms—namely, inhalational, cutaneous, and gastrointestinal. Inhalation anthrax occurs in several discrete steps. Endospores of Bacillus anthracis are taken up by macrophages at the site of initial infection and can be transported to regional lymph nodes. The spores germinate inside the phagolysosome to become vegetative bacteria which can escape from the phagolysosome and replicate within the cytoplasm. Vegetative cells are released into the extracellular milieu and enter the circulation where the vegetative cells grow extracellularly to levels as high as 108 bacteria per ml of blood. In this environment, the vegetative bacteria respond to physiological body temperature and CO2 levels to transcriptionally activate genes responsible for capsule formation and toxin synthesis. Finally, massive edema and organ failure are produced as a consequence of toxin formation. Experience with the 2001 bioterrorism incident found that once the disease reaches the phase where patients show evidence of significant toxin production, treatment with antibiotics can do little to prevent a fatal outcome. Similar results were reported in animal models. Accordingly, early diagnosis and intervention prior to toxin production is essential to patient survival.
Bacillus anthracis can also produce cutaneous anthrax or gastrointestinal anthrax. Cutaneous or gastrointestinal anthrax may show local signs and symptoms. In some cases, cutaneous or gastrointestinal anthrax can disseminate to produce the sepsis syndrome that occurs following inhalation anthrax.
Treatment of anthrax is dependent on administration of antibiotics early in the course of disease. Successful treatment requires that the bacterium be sensitive to available antibiotics and that antibiotics be administered before large amounts of toxin are released. A delay in antibiotic treatment may substantially lessen chances for survival. If a sufficient level of toxin production occurs, there is little in the way of specific therapy that is available for treatment. Currently, bacteriological culture is the mainstay for diagnosis of anthrax. Unfortunately, a preliminary diagnosis of anthrax requires 12-24 h of culture, and definitive diagnosis requires sophisticated assays that are performed by one of the members of the Laboratory Response Network. As a consequence, there is an urgent need for diagnostic tests that will allow for early diagnosis at the point of initial patient contact.
A further complication in the treatment of anthrax is the possibility that a biowarfare strain can be engineered to resist treatment by conventional antibiotics. For example, there is a report of a Bacillus anthracis strain that has been engineered to resist the tetracycline and penicillin classes of antibiotics. Similarly, the bacillus could be engineered to produce a toxin that would evade anthrax vaccines that target the anthrax toxin.
Like many members of the genus Bacillus, Bacillus anthracis is surrounded by a capsule comprised of high molecular weight polymers of glutamic acid. In the case of Bacillus anthracis, the capsule is composed entirely or almost entirely of poly γ-D-glutamic acid (γDPGA). The capsule is believed to contribute to pathogenesis by preventing phagocytosis of the bacterium. This enables the microbe to replicate in blood or tissues at which time the bacterium elaborates three proteins that contribute to the pathogenesis of anthrax—namely, protective antigen, lethal factor, and edema factor.
Studies of γDPGA production during infection and an assessment of protection by anti-γDPGA antibodies have been hampered by the poor immunogenicity of this antigen, the inherent difficulty in generating monoclonal antibodies (mAbs) to weakly immunogenic antigens, and the consequent lack of immunochemical reagents. As a result, neither the extent of γDPGA production during anthrax nor the role of γDPGA as a target for active or passive immunization is known. Recent studies demonstrated that protein conjugates of γDPGA had enhanced immunogenicity in mice, highlighting the γDPGA capsule as a potential target for vaccine development (Schneerson, et al., PROC. NATL. ACAD. SCI. U.S.A., 100:8945-8950 (2003); and Rhie, et al., PROC. NATL. ACAD. SCI. U.S.A., 100:10925-10930 (2003)). However, the effectiveness of anti-γDPGA antibodies in preventing or treating anthrax in vivo has not been reported.